1. Field of the Invention
The invention relates to hydrocarbon refining, and more particularly to a process for removing sulfur compounds from gasoline.
2. Description of the Related Art
The major source of gasoline sulfur (up to 98%) is from the gasoline produced from fluid catalytic cracking (FCC), which comprises 30 to 70% of the gasoline pool. One of the most effective ways to remove the sulfur from gasoline is to hydrotreat the FCC gasoline. However, this stream contains significant amounts of olefinic compounds, and hydrotreating these compounds substantially reduces the octane rating of the blended gasoline.
The typical current approach is to fractionate the FCC gasoline into a light fraction containing non-thiophene type sulfur compounds and hydrocarbons boiling below the boiling point of thiophene (84xc2x0 C.), and a heavy fraction containing all the thiophene-type sulfur compounds and heavier hydrocarbons. The light fraction is then treated in a caustic washing unit (such as a Merox unit) to remove the non-thiophene type of sulfurs. The heavy fraction is fed to a hydrodesulfurization (HDS) unit to eliminate the thiophene type of sulfurs. All olefins which have boiling points higher than thiophene are subject to HDS treatment, resulting in a reduction of octane rating.
U.S. Pat. No. 4,053,369 discloses a two-liquid phase extractive distillation process for the separation of aromatics and non-aromatics which extracts sulfur compounds in the process. However, the disclosure of the above patent is limited to extractive distillation operated with 2 liquid phases in the extractive distillation column.
This invention is related to the incorporation of an extractive process into refining processes to simultaneously extract sulfur compounds and reject olefinic compounds in the hydrocarbon streams. Particularly preferred streams for use with the invention are derived from, for example, a coker naphtha source, a thermal steam cracked source or a fluid catalytic cracker (FCC) unit. Gasoline from a FCC unit is particularly preferred for use with the invention.
The gasoline stream may comprise single and multi-ring aromatics, single and multi-ring naphthenes, olefins, paraffins, thiophenes, benzothiophenes, sulfides, disulfides, thiols, tetrahydrothiophenes, and dihydrobenzothiophenes, having boiling points ranging from about 50xc2x0 C. to about 250xc2x0 C.
Preferably, a prefractionation column is used to remove benzothiophenes and high molecular weight sulfur compounds from said gasoline stream, and the boiling points of the gasoline stream range between about 50xc2x0 C. and about 220xc2x0 C.
According to the invention, only the extract stream with the sulfur concentrates is hydrodesulfurized with a conventional or improved HDS (hydrodesulfurization) unit. In this way, the octane rating of the desulfurized FCC gasoline can be preserved, since the olefinic compounds with higher octane rating are rejected by the extractive process from the stream, which is treated in the HDS unit.
A process to remove sulfur compounds from a gasoline stream containing olefins and sulfur compounds according to the invention comprises subjecting a gasoline stream to an extractive process to concentrate the sulfur compounds in an extract stream and reject olefins to a raffinate stream, and subjecting only said extract stream to hydrodesulfurization to remove sulfur compounds.
In particularly preferred embodiments the process according to the invention comprises an extractive distillation process conducted in an extractive distillation column substantially without a two-liquid phase region.
The selection of the operating parameters of an extractive distillation column, including the appropriate pressures, temperatures, reflux ratios, and solvents used, to avoid a two-phase region is within the skill of the ordinary artisan.